In an alkaline manganese battery, a Ni-plated steel sheet is generally used as the material for a container (positive electrode can) which is used for filling a positive electrode material, a negative electrode material, an electrolyte solution, etc., and which is also used as a positive electrode terminal. Conventionally, Ni plating has been performed by so-called barrel plating method in which plating is performed after the can has been formed. This conventional method may have problems such as insufficient adherence of the Ni-plating layer to the internal surface of the can, or instability in the quality of the plating layer. Accordingly, such method may be replaced by a method in which a pre-plated steel sheet is processed into the form of a can. However, this further method which uses a pre-plated steel sheet is also problematic in that, as the Ni plating layer has high hardness and low extensibility, the pre-plated steel sheet has poor press workability and the plating layer tends to be separated during the processing, thus leading to deterioration in corrosion resistance. In order to resolve this problem, another conventional method may be employed in which heat treatment is performed after the Ni plating to form a Fe—Ni diffusion layer in the interface between the Ni plating layer and the iron substrate so as to improve the close adherence of the Ni plating layer, as well as to improve the extensibility of the Ni plating layer by recrystallization and softening of Ni. The use of this method has led to a significant improvement of the press workability and corrosion resistance.
In order to attain high capacity in an alkaline manganese battery, it is preferable to increase the content of manganese dioxide in the positive mix. However, such increase may lead to an increase in contact resistance due to the low electrical conductivity of manganese dioxide itself. As a countermeasure, a conductive film is formed on the internal surface of the positive electrode can. With such alkaline manganese battery, although the contact resistance is initially low, the contact resistance increases rapidly after prolonged storage at high temperature, thus leading to a degradation of the battery characteristics.
Examples of improvements in the above-mentioned battery characteristics, from the side of the steel sheet material, may be found in the following patent publications.
For example, Japanese Patent Publication No. 05-21044 describes that, as material for DI drawing and ironing, hard plating such as would produce cracks in Ni plating layer during the processing is effective in causing the surface area in contact with the positive electrode material to be increased by the cracks produced during the processing and permitting the battery characteristics to be thereby improved. Various examples of hard plating are described therein such as Ni plating containing organic additives, the aforementioned plating applied via a Fe—Ni diffusion layer, and the like, are described.
Japanese Patent Publication No. 07-122246 and 07-300695, International Patent Publication WO 95/11527, and/or the like, describe a method for ensuring sufficient contact with the positive electrode material by forming a very hard alloy (such as Ni3Sn, Ni3Sn2, Ni3Sn4, etc.) plating layer on the top layer corresponding to the inner surface of a positive electrode can, and producing cracks in the plating layer during press working.
In addition, Japanese Patent Publication No. 08-138636 discloses a method for increasing the surface area in contact with the positive electrode material, and thereby improving the battery characteristics, in which Sn and Ni are plated as dual plating layer, in this order, onto a steel sheet and disposing the plating layer converted into an alloy layer by heat treatment so as to serve as an internal surface of the positive electrode can. In this manner, surface cracks may be produced during press working due to the difference of extensibility between the upper layer being composed mainly of Ni and the lower layer being composed mainly of Sn.
Japanese Patent Publication No. 09-306439 describes a method in which an Ni alloy plating layer having different hardnesses within the plating layer is formed such that the plating layer to be used for the internal surface of a can has higher hardness, for increasing the surface roughness of the internal surface of a can during press working so as to improve the close adherence to the positive electrode material. As an exemplary method for varying the hardness in an alloy plating layer, varying the type or the amount of the metal to be alloyed with Ni or varying the amount of an organic additive, is disclosed in this publication as well.
Further, in Japanese Patent Publication Nos. 10-172521 and 10-172522, a method is disclosed in which a Ni—Co alloy plating layer is formed, or Ni—Co alloy plating is applied via a Ni plating layer. As the Ni—Co alloy is very hard, very fine cracks may be produced in the plating layer during the press working and a very fine roughness is thereby formed so as to improve the contact with the positive electrode material, to thereby improve the battery performance.
Japanese Patent Publication No. 11-102671 describes a method in which Ni—Ag alloy plating or Ni—Cr alloy plating is applied via a Ni plating layer onto the surface to be used as the internal surface of a positive electrode can. Since both the Ni—Ag alloy plating layer and the Ni—Cr alloy plating layer are very hard, very fine cracks may be produced in the plating layer during press working and a very fine roughness is thereby formed so as to improve the contact with the positive electrode material, to thereby improve the battery performance.
Additionally, in Japanese Patent Publication Nos. 11-329377 and 11-329378, a method is described to utilize a Ni—Bi alloy plating layer and a Ni—In alloy plating layer, respectively, to improve the alkali resistance of the aforementioned Ni—Sn alloy plating layer having low alkali resistance, to thereby improve the battery performance.
The various methods in prior art as described above all aim to form a very fine roughness on the internal surface of a can during press working, and for such purpose, a steel sheet having a hard plating layer formed thereon is mainly used to produce fine cracks in the plating layer during press working. However, there may be a problem associated with the concept of producing cracks in the plating layer during press working that the production of cracks in the plating layer varies depending upon the variation of the condition of the press working, and therefore, stable battery characteristics may not be obtained.
In order to overcome the above-described problem, another method is provided and disclosed in International Patent Publication WO 97/44835 for increasing the close adherence to the positive electrode material by applying an electrolytic treatment in acidic solution after Ni plating and thereby roughening the surface. Japanese Patent Publication No. 2000-192281 describes a method in which, after discontinuously forming Ni plated layers, etching is performed in an acidic solution and, further, a Ni plating layer is formed to obtain a surface having many small pits formed thereon. However, these methods require equipment for treatment in an acidic solution, and are therefore undesirable in terms of cost.
It is an object of the present invention to overcome the above-described problems and to provide a plated steel sheet material to be used as a positive electrode can for an alkaline manganese battery and a positive electrode can for an alkaline manganese battery having good battery characteristics.
All cited references are hereby incorporated herein by reference in their entireties.